1. Field of the Invention
This invention relates to a magnetic disc incorporated in a Winchester disc unit or the like wherein reading/writing data and reading servo data for positioning a disc are simultaneously executed.
2. Description of the Related Art
At present, aluminum alloy is put to practical use for material of a substrate of a magnetic disc housed in a Winchester disc unit. However, glass, ceramics, resin, etc. are also proposed for the material so as to realize a smooth-faced magnetic disc lighter in weight.
FIG. 1 is a diagram showing the structure of a conventional magnetic disc made of the material mentioned above and provided with recording layers on both sides thereof, as well as the relation between the magnetic disc and magnetic heads. A magnetic disc 1 of FIG. 1 is comprised of two magnetic layers 3 for recording data therein, and protective layers 4 for protecting respective magnetic layers 3 provided at both surfaces of a glass substrate 2. A magnetic head 6 having a very small gap 9 is arranged adjacent to each recording layer of the magnetic disc 1. A coil 7 is wound around the magnetic head 6.
The principle how the magnetic heads 6 write/read data to/from the magnetic disc 1 will be explained hereinafter. In writing data, the current is supplied to the coil 7, thereby exciting the magnetic head 6 to generate a magnetic flux 8. As a result, leakage flux generated in the small gap 9 magnetizes the magnetic layer 3 in a desired direction. An arrow designated by numeral 10 in FIG. 1 indicates the direction of magnetization of the magnetic layers 3.
On the other hand, in reading data, magnetic fields brought about from the magnetic layer 3 magnetized in specific directions induce currents on the coil 7. These induced currents are read as electric signals of the data.
The magnetic disc 1 having the aforementioned structure is manufactured as follows. In the first place, the disc-shaped glass substrate 2 is formed with the average surface roughness not exceeding 0.2 .mu.m with an outer diameter (.phi.) of 95 mm and a thickness (t) of 1.2 mm. Then, 2000 .ANG. Cr layer as a foundation of the magnetic layer 3, and 600 .ANG. Co alloy layer as the magnetic layer 3 and 500 .ANG. carbon layer as the protective layer 4 are successively formed through sputtering on the glass substrate 2, whereby the magnetic disc 1 is completed.
FIG. 2 is a block diagram showing general structure of a Winchester disc unit. A plurality of magnetic discs 1 are fixed to a common spindle 11 at their respective centers, so that the discs 1 are simultaneously rotated by the rotation of the spindle 11. The opposite surfaces of each magnetic disc 1 are rendered data surfaces 17 for reading/writing data. A data head 12 which reads/writes data from/to the magnetic layer 3 is arranged adjacent to said data surface 17. One of the discs 1 has its one surface assigned for a servo surface 18 wherein servo data for positioning the magnetic head 12 on the magnetic disc 1 is written. A servo head 13 is provided adjacent to the servo surface 18 to read the data from the servo surface 18. The data heads 12 and servo head 13 are respectively supported by head arms 14 which are held by a slidable actuator 15. A voice coil motor 16 slides the actuator 15 to a desired position on the basis of positional data obtained by the servo head 13. Consequently, the servo head 13 and all of the data heads 12 are moved in a radial direction of the magnetic discs 1.
Meanwhile, positioning of the data head 12 in the Winchester disc unit will be discussed hereinafter. When the servo head 13 reads and amplifies positional data preliminarily written in the servo surface 18, the voice coil motor 16 is controlled thereby to slide the actuator 15 to a predetermined position, so that the data heads 12 are positioned at the desired position.
Positional signals as servo data are written by a servo writer to one surface of the magnetic disc 1 fabricated in the foregoing manner. The magnetic disc 1 is then tested by a disc tester incorporating the magnetic disk 1 therein, the result of which will be described below. It is to be noted here that the data heads 12 and servo head 13 are monolithic heads formed of Mn-Zn ferrite.
When only the servo head 13 is installed, the positional signals are properly read.
When the servo head 13 is provided and also the data head 12 is mounted at the disc surface opposite to that the servo head 13 is provided at, while data is being written to the magnetic disc 1 by the disc head 12, the servo head 13 cannot properly read the positional signals, and the data head 12 and servo head 13 are deviated from the desired position.
In the above state, while supplying the data head 12 with current for writing to cause leakage flux, an electromotive force induced to the coil 7 of the servo head 13 is measured, which is large enough to influence induced electromotive force generated to read the positional signals.
In the Winchester disc unit, the magnetic heads 12 and 13 are positioned concurrently with writing/reading of data to/from the data surfaces 17 by the data heads 12. Since electromagnetic waves generated by the data head 12 while writing the data, penetrate the magnetic disc 1 and are to be electromagnetic noises against the servo head 13 reading the positional data, the conventional Winchester disc unit is inconveniently low in positioning accuracy.